Italian authorities have clamped unprecedented security coverage over the Vatican and other Christian landmarks after receiving a "credible threat" of a Christmastime terrorist attack, senior U.S. officials said Wednesday.

Over the past two days, officials within the Vatican and elsewhere in Italy have increased security near major Christian symbols, including St. Peter's Square. The step was taken based on a warning from the Mossad, the Israeli security service, that there was a "high index of probability" that "an important symbol of Christianity" would be the target of an attack between Christmas and New Year's, senior officials told NBC News on condition of anonymity.

Italy has increased security around 8,000 or so sites, which are guarded by a 12,000-member security force, Interior Minister Giuseppe Pisanu told Agence France-Presse.

"There is a legitimate concern, a credible threat," one of the U.S. officials told NBC's Robert Windrem. "Italian police are taking the appropriate precautions."

The phrase "credible threat" has a specific meaning in the intelligence community, denoting information developed from a source or sources that have proved reliable in the past.

Security fears have risen worldwide as the Christian holiday season approaches with tens of thousands of U.S. troops occupying Iraq, a major Muslim nation. Earlier Wednesday, the State Department urged nonessential U.S. diplomats and the families of all U.S. officials to leave Saudi Arabia.

Preparations in Italy
U.S. officials would provide no further details of the alert in Italy. An Italian police official, however, described the threat as more credible than specific, meaning there was more information on the type of attack than on the specifics of where or when it would occur.

Officials in the northern Italian city of Pisa tightened security around its famous Leaning Tower, where a restricted zone is usually flouted by private vehicles, a spokesman for the city police said Wednesday.

"There has been no specific threat against Pisa, but the measure was taken in the current historical context," the spokesman, Massimo La Camera, said without elaborating.

Special attention focuses on the Vatican. The Via della Conciliazione, the main road leading to St. Peter's Basilica, will be closed every night until Dec. 25, a precaution that officials said was unprecedented.

A U.S. official told NBC News last week that an increase in terror warnings was to be expected around the holiday season. "The chatter goes up around their holidays and our holidays," the official said, referring to Muslim radicals.

Saudi warning
In addition to nonessential diplomats and diplomats' families, the State Department also urged private U.S. citizens to consider leaving Saudi Arabia, as well. Americans planning to visit the country were advised to defer their travel because of "the potential for further terrorist activities."

"The U.S. government continues to receive indications of terrorist threats aimed at American and Western interests," the State Department said in a written statement.

U.S. officials acknowledged that orders authorizing the departures of any embassy employees and relatives were not issued lightly. For now, however, the decision to leave the country is voluntary and will be re-evaluated in 30 days, they told NBC's Tammy Kupperman on condition of anonymity.

U.S. officials said the order was not triggered by a specific new threat. Rather, they said, it was based on an appraisal of overall security and the "persistent" general threats in Saudi Arabia, the United States' closest ally in the Arab world, where Muslim radicals launched strikes that killed 26 people at three Western-oriented housing projects in May.

A housing compound in Riyadh was bombed last month, leaving 17 people dead and more than 100 others injured.

By MSNBC.com's Alex Johnson with NBC's Tammy Kupperman in Washington and Robert Windrem in New York.